Nevertheless, and contrary to popular perception, Romney's effective federal income tax rate is still above that of many Americans -- 80% of whom have an effective rate below 15%. That tax rate is higher when other federal taxes -- such as the payroll tax -- are included.

The reason Romney's rate is so low -- despite having one of the highest incomes in the country -- is because his income was derived almost entirely from capital gains and dividends from his extensive portfolio of investments. And that form of investment income is typically taxed at just 15%, well below the 35% top tax rate for high earners.

Another reason: He had extensive itemized deductions, which lowered his overall tax liability.

Romney and his wife, Ann, gave away more than $7 million over the past two years -- or about 16% of their income. Of that, $4.1 million went to the Church of Jesus Christ of Latter-day Saints, to which they belong.

His investment income: Romney took in $21.7 million in long-term capital gains over the past two years. Of that, $12.9 million was in so-called carried interest.

Carried interest is a share of profits paid to general partners at private equity firms -- such as Bain Capital, which Romney left in 1999. General partners manage the firm's investments. If those investments are sold at a profit, the carried interest represents a portion of those profits above a minimum rate of return.

Romney's retirement package from Bain entitled him to continue to share in profits from the firm's work, according to his financial disclosure form on file at the Federal Election Commission.

Romney has been under pressure for weeks by questions of whether and when he would release his tax returns.

The Romney campaign released more than 500 pages of tax documents on Tuesday, including estimates for 2011. The returns were prepared by accounting giant PricewaterhouseCoopers, the Romney campaign said.

Newt Gingrich, his main rival for the GOP nomination, last week revealed his 2010 tax form. He made $3.2 million and paid an effective tax rate of 31%.

Opponents said voters deserved to know early if there were any financial issues that could put the presumptive nominee's bid for the White House in jeopardy.

Romney, whose financial disclosure form puts his net worth as high as $264 million, is one of the wealthiest candidates in history to seek the U.S. presidency.

On a call with reporters Tuesday, Romney campaign advisers pushed back against reports that Romney had sought to avoid taxes by putting some of his money in the Cayman Islands.

Brad Malt, a lawyer who serves as trustee of the Romneys' blind trusts, said Romney pays all U.S. taxes on income from the trusts' foreign investments. Further, he said Romney has no role in choosing how the blind trusts invest his money.

"The blind trust investments in the Cayman funds are taxed exactly as if Gov. Romney owned his share of the funds in the United States," Malt said.

Correction: An earlier version of this article incorrectly stated the two-year effective tax rate by using Romney's 2010 rate alone.